The Sun

Solar Eclipses

During the last few years Shadia Habbal has organized several expeditions to observe total solar eclipses. These events offer rare opportunities to study the solar corona — the region of hot gas that is the source of the solar wind that strongly affectsconditions in the upper atmosphere of the Earth.

By acquiring simultaneous images of iron atoms at different levels of excitation, Habbal and her colleagues established for the first time that

The electron temperature in coronal structures has a clear demarcation, with open field lines associated with the solar wind outflow being characterized by a 1,000,000 K plasma, while the archlike structures are at 2,000,000 K.

The hottest structures enshroud the coolest material in the corona in the form of suspended or anchored prominences.

the empirical determination of the location where the ions become frozen into the charge state that is subsequently measured in interplanetary space occurs within 0.5 to 1 radius above the solar surface.

Localized enhancements of the ion relative to the electron density occur in distinct magnetic structures, thus providing the first empirical evidence for what had been found in models of the solar wind when the heating of heavy ions was quenched.

An overlay of white light, Fe XIV 530.3 nm (green), and Fe XI 789.2 nm (red) emission from observations taken during the total solar eclipse of 1 August 2008.

The Diameter of the Sun

Jeff Kuhn is a member of an international team which has recently measured the diameter of the Sun with unprecedented accuracy.

The used the SOHO spacecraft to time the transits of the planet Mercury across the face of the Sun in 2003 and 2006.
They measured the Sun’s radius as 696,342 km (432,687 miles) with an uncertainty of only 65 km (40 miles).

Transits of Mercury occur 12-13 times per century providing astronomers with the opportunity to look for very small changes in the Sun's diameter over the next few decades.

Mercury's path across the solar disk as seen from NASA's Solar and Heliospheric Observatory on November 8, 2006.